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Can anyone help with the wiring for dual tanks

I aquired a '77 Scottsdale, 4x4, short bed, 350, with dual tanks and at the selector valve underneath the truck it has a wire disconnected. The fuel gauge does not work, neither does the tank selection switch in the cab. It has one wire connected to each tank for the sending units, and one going to the selector valve (which has 12v), the one thats not connect is either a very light pink or tank (no stripe's that I can tell). I've searched everywhere I can think of and looked all over the net, but can't locate a diagram on the wiring for it. Wyldman, responded sometime back about someone intstalling a dual tank setup under a truck with a single tank originally and would send them the diagram for the wiring. So hopefully one of you can help, it would be greatly apprieciated.
Thank you,

I have the wiring diagram at home in the book for my '75, but the '77 is probably the same. However, I'm at work and all this is from memory.

The tan colored wire is the sending unit wire.
The wire goes from the instrument panel through the firewall and then down behind the distributor and across to the passenger side of the truck. From there it goes down the firewall and follows the frame rail back to the passenger side gas tank (which is the main tank).

If you have dual tanks, there is a connector in the tan wire, inside the frame rail back near the gas tank. This is where the dual tank switch wire connects in to the passenger side tank and to the gauge wire.

From the dash switch you have five wires.
One goes to power (red with white?) at the fuse block
One runs back to provide power to the tank selenoid
One tan wire that goes to the gauge
Two (tan/white, I think) that go to the sending units

Four of the wires from the switch go through a rubber plug in the firewall, then extend down the firewall to the driver's side frame rail.
One runs back along the frame rail and connects to the selenoid
One tan/white to the driver's side sending unit
One tan/white to the passenger's side sending unit - this wire is much longer because it runs down the driver's side, then across the frame to the passenger's side
One tan to the gauge - this is also long, because it runs across the frame and connects to the tan gauge wire that is on the driver's side frame rail.

If you can follow my explanation above, you realize that the tan gauge wire has a very convoluted path from the gauge, through the switch, and back to the tanks.

Hey it would be great if you post the wiring diagram. If you send it out individually, I'd like a copy. I'm going to remove the bed on a C30 3+3 tomorrow to remove the tanks, saddles, wires & plumbing for my 77 K20. It would be a great help. They cut the under hood wires at the junk yard.

I'll see if I can get my scanner to work on the diagrams. If not, they are simple enough that I can probably just draw it up and post it.

I found two fuel tank wiring diagrams in my GM repair manual. One matched my truck and the donor truck, but the other has a completely different switch and wiring. I found an '81 in the junkyard that matched my '75, so I can't figure out the pattern of which trucks use which wiring. I'll post both diagrams.

You probably want to get the donor truck's metal crossover lines when you pull the tanks. This means you only need very short rubber lines to connect to the tanks.

Some trucks use a 6 port fuel selenoid/switch and others use a 3 port. You have an HD truck, so it probably can use a 3 port.

Someone had mentioned that one of the wires were to go to the gauge. In the cab floor pan there are two holes (not rust) and on the passenger side someone had a piece of wire pig tailed to another wire that ran along under the carpet to the driverside under where your feet would sit. So if thats the one disconnected is to return to the gauge cluster, anyone have a clue as to were. Being that I have the cluster loose to check for just that, but cannot find a cut or loose wire, and after following the run from the gauge back to the plug it does have a tan wire there, but it goes down to the plug for the ignition switch and thats were I lose the wire. Being that I do not wish to dismantle the steering collumn as well. Thanks for the suggestion's so far. And it would be a great thing to post the diagram somewhere for others in the future.

The wiring diagram is from my '75 manual, but it also matched the donor '81 that I got a fuel tank and switch from.

I found a couple of trucks in the junkyard that had different tank wiring than mine, but I didn't check them out carefully. I did notice that they had fewer wires through the firewall and fewer wires connected to the dash switch.

My book shows two other possible wiring diagrams, one for a dealer installed aux tank and one that also says RPO/NL2.

Thanks alot for the help the diagram was right on the money, now gauge works for both tanks. With only one slight problem, with the headlights, and radio on the fuel gauge whips about a quarter inch. But, that I can accept much better than one that doesn't function at all. Once again thanks a ton for all the help you guys are great.
Sincerely,

I've got two different diagrams of how to wire these tanks, one of which is posted in this thread. Previous owner installed a second tank and said the gauge works, but needs wired up, all wires are tagged. But the tags are not readable and the way it looks like a cross between the two diagrams. The first part of the circuit coming off the control switch has 4 wires running out the firewall and down to the tanks....but when you get down to the selector valve/meter switch, i've got 5 wires coming out of it...much like a different diagram where there should only be 2 wires coming out from the firewall from the control switch....i'm very confused and don't know if anyone can help me, given the confusing info i've given...

I'm confused about what you mean by control switch vs. selector valve/meter switch. The solenoid under the truck should only have one wire connected, which is the 12v power wire. However, I'll try to explain the wiring.

The dash-mounted switch has five wires:
Four go through the firewall: left tank sender, right tank sender, fuel gauge (this wire goes through the firewall, into the engine wiring loom, and then back through the firewall to the gauge), and the 12v power to the selenoid under the truck.
The last wire connected to the switch is the 12v power feed, which leads over to the fuse box.

The dash switch is a double pole, double throw switch.
-- One half of the switch (or pole) has three terminals, with the gauge connected to the center terminal. "Throwing" the switch up or down connects the gauge to either the main (right on my '75) or aux (left) tank sender.
-- The other half of the switch has three terminals, with 12v connected to the center terminal. If the switch is thrown to main tank it does not connect anything (the center is connected to the open terminal); if it is set to aux, the center terminal (12v) is connected to the line that runs to the solenoid under the truck. This activates the solenoid and switches the lines to the aux tank.

Bruce

bowtie726 said:

I've got two different diagrams of how to wire these tanks, one of which is posted in this thread. Previous owner installed a second tank and said the gauge works, but needs wired up, all wires are tagged. But the tags are not readable and the way it looks like a cross between the two diagrams. The first part of the circuit coming off the control switch has 4 wires running out the firewall and down to the tanks....but when you get down to the selector valve/meter switch, i've got 5 wires coming out of it...much like a different diagram where there should only be 2 wires coming out from the firewall from the control switch....i'm very confused and don't know if anyone can help me, given the confusing info i've given...

ok thanks, now I understand the switch... but between the tanks under the truck there is not just a soleniod, it's something else. It has plug with 5 wires coming off it, although none of them are hooked up to anything. And they're labled on the plug as A,B,C,D, and E. So the only thing I can think of is that someone tried to hook up that other gas tank by putting on that big "fuel tank selector valve", as it is called in one schematic, when they should have used the soleniod, as i see used in a different schematic. So I guess, what I'm going to do is apply 12v to each of the 5 wires coming from the selector valve and see if doing so will switch over to the other tank, and then I'll just wire it up as if it were just the soleniod and ignore the other 4 wires....I've attached the two wiring diagrams I've found that I'm working off of....the first one shows the use of a soleniod with 4 wires coming from the firewall, the second shows the use of the fuel tank selector valve with the 5 wires coming from it, and only 2 from the firewall. I guess they're just different years....but I'm somewhere in between with 4 wires coming from the firewall and 5 going to the selector, take a look!

I picked up a replacement solenoid at the junkyard last month, and there were a lot of dual tank trucks in the yard. All of them had a similar solenoid arrangement, so I just picked a truck that had a six port solenoid that would switch both the fuel feed and fuel return lines.

I don't know exactly what I paid for the solenoid, since they grouped it with some other odd parts I pulled, but I wouldn't think it would cost more than about $10. Switching to a standard setup might be easier than figuring out what you already have.

Alright, trying to find a solenoid to replace the thing I've got under there... The parts store said they've got one type that is for use with return lines, and one that isn't... so if I were to get one without return lines, I take it I'd have to set it up so it would always return to one tank? And just plug the line that's supposed to be a return for the other tank? Or should you T the return line so it just returns to both tanks?

My original setup was a dealer-added aux tank, and it used a three port solenoid and a T in the return line. However, it always seemed to return to only one tank, so sometimes it seemed that I was filling one tank from the other through the return line. This made it very hard to figure out gas mileage. I switched to the six port solenoid last month because it should make it easier to keep track of how much gas I have in each tank.